But it wasn't a "final battle." It was Starfleet's primary engagement of the Borg. The only reason why you would take a ship out of mothballs is as a last resort because all your other ships have been destroyed. That's not what happened here.

Whether destroyed or distant, the supposed other starships would have been equally unavailable. We did see some antiquities in the Wolf 359 collection of ships; speculating that they were activated to compensate for the lack of active-service ships near Earth helps accommodate the oddball designs.

In addition to or instead of museum pieces, these designs could also have been one-off prototypes or test rigs, found in abundance at the busy shipyards of Sol - this would match the real-world identity of many of the models.

it seems that Starfleet didn't consider them to be that much of a threat if that's all they sent

But that interpretation seems unsupportable, as the fleet of forty was mentioned in the same breath with pleas for Klingon and even Romulan assistance. Forty'ish is all they could manage, and that was apparently with scraping the bottom of the barrel.

As for the Connie at Wolf 359, it's never been canonically shown that it was a Connie

Very true. There's room for the Belknap class in canon yet, if we want to.

And if you're referring to the Tripoli from "Unification," I was always under the impression that the Romulans or whoever was piloting that raider ship simply tractor beamed or just destroyed the ship before taking it's place, not that they reactivated it and flew it away.

Sounds like a much "noisier" alternative, one less likely to go under the admittedly low-grade radar of the Zakdorn gatekeeper.

Also, there is no onscreen evidence to suggest that the Mirandas existed in the time before TMP.

...Other than perhaps the existence of a 1800-range registry in "Court Martial" already.